Server alarm in HPSIM for no apparent reason

We have a server running in a VM: ESX 3.5.0, Windows 2000 SP4. which has been regularly alarming in HPSIM with "System is unreachable", yet there is nothing in the OS event viewer which points to a cause for the alarm. A few minutes after the alarm we get another message advising "System is reachable". This happens 1-2 times per day.